Polarisers for very high frequency electro-magnetic waves



Nov. 22, 1966 J. R. MARK ETAL. 3,287,729

'POLARISERS FOR VERY HIGH FREQUENCY ELECTRO-MAGNETIG WAVES Filed Nov.26, 1962 W .37 mz #4 ATTO RNEYS United States Patent 3,287,729POLARISERS FOR VERY HIGH FREQUENCY ELECTRO-MAGNETIC WAVES John RichardMark and Ronald Kenneth Walker, Chelmsford, England, assignors to TheMarconi Company Limited, a British company Filed Nov. 26, 1962, Ser. No.240,038 Claims priority, application Great Britain, Dec. 14, 1961, 44,872/ 6 1 4 Claims. (Cl. 343756) This invention relates to polarisingarrangements for very high frequency electro-magnetic waves and moreparticularly to polarising arrangements providing, at will, eitherlinearly or elliptically polarised waves. The term elliptically is usedin this specification in a broader sense than is customary to includecircularly which is regarded, in this specification and from theviewpoint of this invention, as a special case of elliptically. Theterms polarising arrangements and polarisers as used herein includearrangements or devices which will serve to affect the direction and/ ornature of the polarisation of electormagnetic waves.

It is common practice in radar and other microwave systems to use inconjunction with a radio reflector a tapered radio horn, fed from asuitable waveguide system. It is also a common requirement that saidhorn should be capable of providing, at will, either linearly orcircularly polarised radiation. Such horns are, of course, normally oftapered rectangular section and, in designing the horn, the aspectratioi.e. the ratio of length to breadth of the horn aperture-is chosento suit the reflector with which the horn is to be used so as to providebest illumination of the reflector. With such rectangularly sectionedhorn, the taper is not usually the same in the two mutuallyperpendicular planes which are parallel, respectively, to the directionsof length and beadth of the horn aperture. cause of this, when such ahorn is employed for circularly polarised waves, it introduces differentphase shifts for the two mutually perpendicular components of thecircularly polarised wave. It is customary to correct for phase errorarising in this way by providing in a rectangularly sectioned waveguideleading to the horn a suitable compensating element which introduces therequisite compensating phase shift between the two mutuallyperpendicular components of the circularly polarised wave. Such acompensating element may consist, for example, of a metal or dielectricvane projecting into said Waveguide from one wall thereof. There isusually provided, leading to the guide length containing thecompensating element, a polariser, consisting, for example, of a lengthof rotatable circularly sectioned guide with a suitably shapeddiametrically situated dielectric vane in it, and adapted to provide, independence on the position of the circularly sectioned guide, either alinearly or circularly polarised wave. Thus a typical known arrangementwould comprise in the order stated (a transmitting arrangement isassumed here though, of course, the same arrangement will serve forreception) a rectangularly sectioned feeder fed with linearly polarisedwaves; a circularly sectioned rotatable polariser containing adielectric vane or other known polariser element and fed from the feedervia a rectangular-to-circular transistion guide length of known form; arectangularly sectioned phase compensator containing a phasecompensation element and fed from the polariser via acircular-to-rectangular transition guide length of known form; and atapered radio horn fed from the compensator. In this type of arrangementthe polariser has a limited frequency bandwidth over which it willoperate satisfactorly and where the polariser and compensator are sodesigned as to produce a correctly Bev "ice circularly polarised wave atthe mouth of the horn, at one predetermined frequency, the phase errorsproduced by the polariser at other frequencies will not be compensatedby the compensator due to the fact that, when arranged to providecircularly polarised waves, the dielectric vane of this polariser is atan angle to the sides of the horn and the waveguide incorporating thecompensating element. Consequently the known arrangement suffers fromthe defect that it will operate satisfactorily over only a relativelynarrow band of frequencies. The principal object of the presentinvention is to overcome this defect.

According to this invention a polarising arrangement adapted to provideat will either elliptically (including circularly) or linear polarisedelectro-mangetic from input linearly polarised waves comprises a lengthof circularly sectioned waveguide having an odd number of successivesubstantially diametrically arranged conductor units, said unitsextending substantially across the interior of said guide and arrangedat angles to one another, means, operable at will, for causing saidconductor units to impart an overall change of either 45 or zero in thedirection of linear polarisation of waves propagated through saidwaveguide and means for relatively phase delaying, to an extent toprovide the required degree of ellipticity of polarisation, componentsof the output waves from said waveguide which are respectively paralleland perpendicular to the end conductor unit at the input end of saidwaveguide.

Where it is desired to provide either circularly or linearly polarisedwaves the extent of the phase delay will be 90".

In a preferred embodiment of the invention, said phase delaying meanscomprises a length of rectangularly sectioned waveguide having a phasecompensating element therein and, connected to said waveguide, a taperedrectangular radio horn, one pair on opposite sides of said rectangularlysectioned guide being parallel to said input end conductor unit, andsaid rectangularly sectioned guide and radio horn together providing arelative phase delay of 90 to said wave components.

In one form of embodiment the conductor units are comprised of spaceddiametral metallic conductors, the angle between the central conductorunit and the input end unit being 22 /2 and the output end unit,together with the remaining units (if any) between the central unit andthe output end unit, being rotatable about the axis of the guide so thatthe output end unit may, at will, be brought either parallel or at 45 tothe input end unit. Although each conductor unit may consist of a singleconductor this is not preferred and preferable each unit consists of aplurality of parallel spaced metallic conductors, all the conductors inany one unit being coplanar. Thus there may be, for example, threeequally spaced conductor units each consisting of a number of coplanardiametral conductors, for example, three. It will be seen that when thetwo end units are at 45 to one another the input waves will have theirdirection of polarisation turned through 45 but when the two end unitsare parallel there will be no effective change in the direction ofpolarisation.

The invention is illustrated in the accompanying drawings in whichFIGURE 1 is a schematic perspective view of a polarising arrangement inaccordance with this invention employing a tapered radio horn, andFIGURE 2 shows a form which one of the elements of FIGURE 1 may take. Ineach of the figures the arrow represents the electric vector of theinput linearly polarised wave.

Referring to FIGURE 1, the installation therein generally shown may beregarded as consisting of four units, namely (assuming the installationto be employed for transmission) an input feeder unit generallydesignated A, a polariser unit generally designated B, a phasecompensator unit generally designated C and a radio horn D.

The input feeder unit consists of a length of rectangular sectionedwaveguide A1 unitarily formed with a rectangular-to-circular waveguidetransistion A2 as known per se. The polariser unit B consists of apolariser B1 and a circular-to-rectangular waveguide transition B2. Thepolariser is represented only in outline by a cylinder in FIGURE 1, butone form which the polariser may take is shown in FIGURE 2. The unit Cconsists of a length of rectangularly sectioned waveguide C1 having itssides parallel to the corresponding sides of waveguide A1, with ametallic or dielectric vane or similar phase compensating element C2 asknown per se. The horn D is a normal tapered horn of rectangular sectionand an aspect ratio which will in normal practice be determined by theshape and arrangement of a reflector (not shown) which the horn is toilluminate. The phase compensating element C2 is so arranged, inaccordance with known practice, that the unit C and horn D incombination provide a relative phase delay of 90 between twoperpendicular wave components which are parallel to adjacent sides ofthe waveguide C1.

FIGURE 2 shows one form of polariser suitable for use at B1. Thepolariser comprises two colinear, coaxial lengths of circular waveguide1B and 2B arranged in any convenient manner so that the length 2B can berotated with respect to the length 1B about the common axis. The twoends of the rotatable length of guide are indicated in FIGURE 2 bybroken line circles.

Within the fixed length 1B are two spaced diametrically arrangedconductor units each comprising of three parallel coplanar diametralmetal tubes or rods which are referenced 1B1 in the case of theconductor unit nearest the input feeder and 1B2 in the case of the otherconductor unit. In the rotatable length of guide is a similar conductorunit consisting of three diametrical parallel coplanar metal rods ortubes referenced 2B1. The units 1B1, 1B2 and 2B1 are equally spacedalong the axis and they are alike. The plane of the unit 1B1 is at 22 /2to the plane of the central unit 1B2. The plane of the unit 2B1 willobviously depend upon the position of rotation of the rotary length ofguide 2B. If circular polarisation is to be produced from the horn D(FIGURE 1) the rotary length of guide is rotated until the plane of theconductor unit 2B1 is at 22 /z to that of the unit 1B2 and 45 to that ofthe unit 1B1. The plane of the said unit 1B1 is at right angles to theelectric vector of the input waves from the input feeder andconsequently the output waves from unit 2B1 will have a direction ofpolarisation at 45 to that of the input waves. If the linearly polarisedinput Waves are merely to be transmitted by the horn without change inthe nature of the polarisation (i.e. as linearly polarised waves) therotatable guide length 2B is rotated until the plane of the unit 2B1 isparallel to the plane of the unit 1B1. This position is shown in brokenlines in FIGURE 2.

It will be seen that when the polariser is set in either of these twopositions it produces two angles of twist as respects the incident wavelinear vector, each being of 22 /z and the total angle of twist being45. Accordingly reflections at the successive conductor units will be ofequal magnitude and may be made to cancel by suitably choosing thespacing of said units in accordance with known principles. In the onecase the two angles of twist are progressive so that there is an overallangle of twist of 45 while in the other case the two angles of twistcancel one another.

Reverting to FIGURE 1 it will be seen that when the polariser B1 isararnged to provide an output wave of the same polarisation as the inputwave, this output wave will be unchanged in passing through thewaveguide C1 and horn D and will produce at the mouth of horn D alinearly polarised wave having a direction of polarisation substantiallythe same as that of the input wave. When the polariser B is arranged toprovide an output wave having a direction of plarisation at 45 to thatof the input wave, however, the two equal and mutually perpendicularcomponents of the output wave will sufier relative phase delay, in theWaveguide C1 and horn D, of and will accordingly provide, at the mouthof the horn, a circularly polarised wave.

It will be seen that in the illustrated embodiment the bandwidth, asrespects the obtaining of true circular polarisation, is determinedsubstantially solely by the Waveguide-horn combination C1-D.

In cases in which the polariser in accordance with the invention is notrequired to provide output waves at the mouth of a radio horn, the hornis dispensed with and the waveguide C1 arranged above to provide therequired phase delay.

We claim:

1. An adjustable polarizer for providing either elliptically or linearlypolarized electromagnetic waves from input linearly polarized wavescomprising first and second colinear, coaxial, circularly sectionedlengths of waveguide, said second length of waveguide being rotatablerelative to said first length of waveguide about the common axisthereof, means for applying linearly polar.- v

ized waves' to said first length of waveguide, a pair of spacedconductor units extending diametrically within said first length ofwaveguide at an angle of substantially 22 /2 degrees to each other, athird conductor unit extending diametrically within said second lengthof waveguide and rotatable therewith between an angular range of plus orminus 22 /2 degrees with respect to the nearest of said pair of spacedconductor units, and means for relatively phase delaying to an extent toprovide the required degree of ellipticity of polarization, componentsof the output waves from said second length of waveguide which arerespectively parallel and perpendicular to the conductor unit which isnearest to the input end of said first length of waveguide.

2. A polarizer as claimed in claim 1 which is adapted to provide at willeither linearly or circularly polarized waves from linearly polarizedwaves and wherein the extent of the phase delay is 90".

3. A polarizer as claimed in claim 2 wherein the phase delaying meanscomprise a length of rectangularly sectioned waveguide having a phasecompensating element therein and, connected to said waveguide, a taperedrectangular radio horn, one pair on opposite sides of said rectangularlysectioned guide being parallel to said input end conductor unit, andsaid rectangularly sectioned guide and radio horn together providing arelative phase delay of 90 to said wave components.

4. An arrangement as claimed in claim '1 wherein each conductor unitconsists of a plurality of parallel spaced metallic conductors, all theconductors in any one unit being coplanar.

References Cited by the Examiner UNITED STATES PATENTS 2,159,937 5/1939Zworykin 343-909 X 2,438,119 3/1948 Fox 333-21 2,735,092 2/1956 Brown343756 3,031,661 4/1962 Moeller etal 343756 3,076,188 1/1963 Schneider"u 343T756 X ELI LIEBERMAN, Primary Examiner,

1. AN ADJUSTBLE POLARIZER FOR PROVIDING EITHER ELLIPTICALLY OR LINERALY POLARIZED ELECTROMAGNETIC WAVES FROM INPUT LINEARLY POLARIZED WAVES COMPRISING FIRST AND SECONE COLINEAR, COAXIAL, CIRCULARLY SECTIONED LENGTHS OF WAVEGUIDE, SAID SECOND LENGTH OF WAVEGUIDE BEING THE ABLE RELATIVE TO SAID FIRST LENGTH OF WAVEGUIDE ABOUT THE COMMON AXIS THEREOF, MEANS FOR APPLYING LINEARLY POLARIZED WAVES TO SAID FIRST LENGTH OF WAVEGUIDE, A PAIR OF SPACED CONDUCTOR UNITS EXTENDING DIAMETRICALLY WITHIN SAID FIRST LENGTH OF WAVEGUIDE AT AN ANGLE OF SUBSTANTIALLY 221/2 DEGREES TO EACH OTHER, A THIRD CONDUCTOR UNIT EXTENDING DIAMETRICALLY WITHIN SAID SECOND LENGTH OF WAVEGUIDE AND ROTATABLE THEREWITH BETWEEN AN ANGULAR RANGE OF PLUS OR MUNUS 221/2 DEGREES WITH RESPECT TO THE NEAREST OF SAID PAIR OF SPACED CONDUCTOR UNITS, AND MEANS FOR RELATIVELY PHASE DELAYING TO AN EXTENT TO PROVIDE THE REQUIRED DEGREE OF ELLIPTICITY OF POLARIZATION, COMPONENTS OF THE OUTPUT WAVES FROM SAID SECOND LENGTH OF WAVEGUIDE WHICH ARE RESPECTIVELY PARALLEL AND PERPENDICULAR TO THE CONDUCTOR UNIT WHICH IS NEAREST TO THE INPUT END OF SAID FIRST LENGTH OF WAVEGUIDE. 